Oscillator-detector circuits



June 16, 1936;

OSCILLATOR DETECTOR CIRCUITS Filed May 18, 1953 Cease: x

n P. o. FARNHAM 2,044,071

Patented June 16, 1936 UNITED STATES OSClLLATOR-DETECTOR CIRCUITS PaulO. Farnham, Boonton, N. J., assignor, by mesne assignments, to RadioCorporation of America, New York, Delaware N. Y., a corporation ofApplication May 18, 1933, Serial No. 671,769

4 Claims.

This invention relates to oscillator-detector circuits such as may beused in receivers in which a locally generated oscillation is combinedwith a received carrier Wave, and particularly to circuits in which asingle tube functions both as the oscillator and the detector, i. e., asa modulated oscillator. Y

Single tube circuits have been previously proposed but have notsatisfied some or all of those requirements which are desirable, if notabsolutely essential, in combination oscillatordetector circuits. Onerequirement is that the oscillator will continue to function as thetranslation gain is adjusted over Wide limits by varying a `directcurrent bias, and a second requirement is that negligible voltage betransferred or induced from the oscillator into the carrier frequencycircuit, i. e. the input circuit of the device.

An object of the invention is to provide an oscillator-detector circuit,including a single tube of simple design, which will satisfy both thestated requirements for circuits of this type. Another object istopprovide oscillator-detector systems including a single tube having aminimum number of elements for effecting a desirable transmissioncontrol without subjecting the radio input circuit to the oscillatorvoltage. More particularly, an object is to provide anoscillator-detector including a tube with three grids, a cathodeandplate; the plate and cathode being including in the oscillator circuit,one grid being associated with the oscillator and another grid with theinput circuit, and these grids being electrostatically shielded fromeach other by an intervening grid.

These and other objects and advantages of the invention will beapparent' from the following drawing, in which:

Fig. 1 is a fragmentary circuit diagram of a superheterodyne receiverembodying the invention, and

Figs. 2 and 3 are curve sheets comparing the characteristics of aconventional amplier tube of the suppressor grid type with a tubeparticularly adapted for use in the novel circuit.

E In the drawing which illustrates the detailed circuit arrangement ofonly the oscillator-first detector stage of a superheterodyne receiver,the reference numeral I identifies a carrier Wave amplifier of anyappropriate design which passes amplified carrier wave to the inputcircuit of the combined function tube 2 which is of the general type ofknown suppressor grid tubes in that it includes threegrids G1, G2, Gabetween (Cl. Z50-20) the cathode K and the plate P. The tube 2 may be avariable-mu pentode, for example of the type known commercially as type58. 1 have found the variable-mu characteristic to be highly desirablein connection with my invention (see Proc. I. R.' E., vol. 18, page2,102, Ballantine and Snow, Reduction of Cross-Talk in Radio, Receiversby Means of Variable-Mu Tetrodes) and in particular, a special type ofvariable-mu characteristic, in which appreciable current flows in theoutput circuit, not only at relativ-ely high values of control-gridbias, but also at substantially zero values/of control-gridtransconductance.

The radio input circuit is connected between grid Gi and the cathode andincludes inductanceV 3, and, preferably, a tuning condenser 4.

The low potential terminal of the inductance is grounded upon thecathode for radio frequencies by the condenser 5 and is returned to thecathode, for direct current, through an adjustable source of biaspotential, as indicated diagrammatically by the Voltage divider 6shunted across the bias battery l. 'I'he intermediate grid G2 ismaintained at the radio frequency potential of the cathode K by theby-pass condenser 8, and is subjected to a direct current potential EQ2by a direct current connection 9 to an appropriate source of current,which is indicated diagrammatically by the battery I0.

The oscillator system includes the grid coil II which is tuned bycondenser I2, the high potential terminal of coil II being connected togrid G3 and the low potential terminal is connected by a direct currentpath including resistance I3 to a direct current source and through theusual series condenser I5 to ground, the condenser I5 being provided toaline the oscillator carrier circuits for simultaneous tuning. Theconnection from resistance I3 to a direct current source, indicated bybattery I6, is preferably through an adjustable device, such as voltagedivider I1, which permits adjustment of the voltage Ecs on grid G3. Theplate coil I8 is coupled to grid coil H, and connected between groundand the plate P through a condenser I9 which may be, and preferably is,adjustable for tuning the plate output inductance 2U to the intermediatefrequency of the receiver. The output coil 20 is grounded for radio andintermediate frequencies by condenser 2I and, as indicated, the platecurrent supply may be completed through coil 2Ilby a connection 22 tothe direct current source IIJ.

' The output coil 20 may be coupled to the intermediate frequencyamplifier in any appropriate or desired manner. As illustrated,transformer coupling may be employed, the tuned circuit 23 passing theintermediate frequency voltage E to the succeeding section of thereceiver, indicated generically at 24.

The design of the oscillator is preferably such that a constantfrequency difference, equal to the intermediate frequency to which theoutput circuit 20 is tuned, is maintained between the resonantfrequencies of the oscillator and radio input circuit when the tuningcondensers are simultaneously adjusted. The tuning element or elements25 of the carrier wave amplifier will usually be mechanically connectedto the other tuning elements to permit use of the usual single controltuning system.

Reverting to the requirements to be satisfied in an oscillator-detectorsystem, it is apparent that the control grid G1 of the radio inputcircuit is shielded from the oscillator voltage by the grid G2 which ismaintained at the same alternating current potential as the cathode. Thecircuit described does, however, present a problem in the maintaining ofoscillator action when the translation gain is substantially reduced bythe application of a heavy negative bias En on the grid G1.

The plate or anode is common to the oscillator and intermediatefrequency circuits and therefore the direct current component of theplate current decreases as the bias voltage En is made more negative. Ihave found that a variable-mu tube in which a relatively high directplate-current flow persists at low Values of control-grid to platetransconductance, is particularly well suited for sustained oscillationat high negative bias voltages.

The curves of Fig. 2 are plotted between negative values of the directcurrent control grid bias En and the direct current plate current ip.

Curve A represents the characteristic of one type of tube having avariable-mu control grid, which is applicable to my invention in caseswhere a limited range of translation-gain control is tolerable, thecontrol being obtained by varying the control-grid bias. Curve B showsthe characteristic of a variable-mu tube' adapted for a wider range oftranslation-gain control. The transconductance at any bias voltage isdefined as the slope of the graph at that bias. It will be noted thatthe tube for which curve B was plotted has higher values of platecurrent, for the same transconductance, than does the tube of curve A.

The significance of the control-grid plate-current curves of Fig. 2 maybe seen by reference to Fig. 3, in which is plotted the amplitude of theoscillating current maintained in the plate circuit, between grid Ga andthe plate, as a function of the control grid bias E01. It is apparentthat the intermediate frequency output voltage E from the device, andhence the translation gain, is proportional to the slope of curves A andB' of Fig. 3. With the type of tube for which curve A (corresponding tocurve A of Fig. 2) is plotted, the plate current becomes substantiallyzero at a moderate Value of negative bias on the control grid, and inthis region the plate-circuit oscillation stops.

The point of cessation of the plate oscillation thus imposes a limit onthe extent to which the translation gain (slope of curve A) may bereduced by Varying this bias. With the type of tube for which curve B(corresponding to curve B of Fig. 2) is plotted, an appreciable currentstill flows to the plate for relatively large values of control-gridbias, out to the region where the slope of curve B is substantiallyZero. Consequently this curve represents a condition which is adaptedfor an extended range of translationgain control by means of the controlgrid bias. It is an experimental fact that with any tube of thevariable-mu type, a favorable operating condition may be realized forthat particular type of tube either between, or at one of, the statesillustrated in Fig. 3, by adjusting the polarizing voltages on otherelectrodes, as for example the voltage Ecs.

The efficiency of the system as a frequency converter compares favorablywith that of the usual separate oscillator and rst detector circuits. Inoperation, a transrectiiication conductance factor of between 300 and400 micromhos has been obtained. This factor is defined as the ratio ofthe anode intermediate frequency current, with no anode load, to carrierVoltage E1 on the grid G1, and is a measure of the merit of the deviceas a converter of carrier-frequency oscillations to intermediatefrequency oscillations.

I claim:

l. A combined vacuum tube oscillator and detector comprising a cathode,a plate and two control grids located at different distances from saidcathode, an input circuit connected to said cath- 0de and one controlgrid, said one control grid being constructed to provide a variable mucharacteristic, means for resonating said input circuit to the incomingsignal frequency, means for feeding back energy from the plate circuitto said second control grid, means for adjusting the frequency of thesaid energy feedback to a frequency differing from the applied signalfrequency by an amount equal to the desired intermediate beat frequency,a screen grid interposed between said control grids and a single meansfor applying a positive potential to said screen grid and a greaterpositive potential to said plate.

2. The arrangement of the preceding claim in combination with acondenser having one side directly connected to said screen grid and itsother side to said cathode.

3. In a modulated oscillator system, the combination of a vacuum tubehaving a cathode and at least four cold electrodes including a signalcontrol grid located adjacent said cathode, of an oscillating circuitnetwork connected between the cathode and two of the cold electrodesfurthest removed from the cathode, an output circuit connected betweenthe cathode and one of the said two cold electrodes, a tunable inputcircuit connected between the cathode and signal control grid, a pathhaving negligible impedance to radio frequency currents connecting theremaining cold electrode and cathode, means for applying a steadynegative bias potential to said signal control grid and means forapplying steady bias potentials to the remaining cold electrodes, saidsignal control grid being constructed to provide a variable mucharacteristic of such form as to reduce to substantially zero thetransconductance between said signal control grid and the farthest coldelectrode.

4. In a modulated oscillator system, the combination of a vacuum tubehaving a cathode, a plate, a signal control grid, an oscillator grid, a.circuit tunable to an incoming signal frequency connected between saidsignal control grid and cathode, means for feeding back energy from theplate to said oscillator grid, means for adjusting the frequency of saidenergy feedback to a frequency differing from the incoming signalfrequency by an amount equal to the desired intermediate beat frequency,a screen grid interposed between said signal and oscillator grids andmeans for applying steady positive voltages to said screen grid andplate, said signal control f grid being so designed with reference tosaid last named means that a direct current of substantial value flowsbetween said plate and cathode when said signal control grid is made sonegative with reference to said cathode as to reduce to substantiallyzero the Value of the signal control grid-plate transconductance.

PAUL O. FARNHAM.

